Conventional inductive-type automotive ignition systems commonly utilize power semiconductor devices to control the switching of current through ignition coils. Such semiconductor devices are typically controlled so as to switch from an “off” state to a fully saturated “on” state within a short time period, whereby such switching results in the voltage across the ignition coil changing rapidly from substantially zero volts to near battery voltage.
A common type of semiconductor device used in this type of application is an insulated gate bipolar transistor (IGBT). These devices switch large currents while being controlled by the modulation of small voltages on the IGBT's control terminal or gate. Positive voltage applied to the gate in excess of the IGBT's threshold voltage causes the IGBT to begin conducting available current through its collector and emitter terminals. Since the coil load switched by the IGBT is inductive in nature, when the IGBT is initially switched on, the inductance of the coil prevents immediate flow of current into the IGBT. This results in a rapid collapse of the voltage across the collector and emitter terminals of the IGBT. The fast change in IGBT voltage appears across the primary winding of the ignition coil. Since an ignition coil is typically a two winding transformer, the change in primary voltage is multiplied by the turns ratio of the primary and secondary windings of the coil, producing a much higher voltage at the coil's secondary output terminal. If the voltage at the coil secondary output terminal is sufficiently high in magnitude, a spark may result across the gap of a connected spark plug at an unwanted moment. The spark event may occur at a time far in advance of the desired ignition time defined by the internal combustion engine piston position resulting in poor engine performance or significant damage.
A number of systems have been employed to prevent a mistimed spark event, including the use of current blocking diodes in series with the coil's secondary winding, and pulsed or phased turn on techniques that use the coil's natural response to reduce the voltage resulting from the IGBT switching event. These systems either prevent secondary current flow altogether or reduce the secondary voltage response such that a spark does not occur. However, these systems result in either additional system cost associated with the diode or increased IGBT gate voltage control circuitry complexity necessary for implementation of the pulsed turn-on method. Conventional systems increase system costs by a non-negligible amount.
Controlling a change in a coil primary voltage in these systems involves determining the actual bias conditions on the IGBT's collector terminal. Direct observation of this voltage necessarily involves managing voltages on the order of several hundred volts at the collector terminal when the IGBT is switched off at the desired spark generation time. Since most conventional systems employ control circuitry implemented in integrated circuit form, using either standard CMOS or bipolar processes, direct application of these high collector voltages results in damage to the integrated circuitry. Voltage divider components external to the integrated circuitry can reduce the voltages to a manageable level, however, voltage dividing networks add system cost and can interfere with other system performance requirements such as limitations on collector leakage currents. Collector voltage control via capacitive feedback methods provides information relating only to the rate of change of the collector voltage and not to the magnitude of the collector voltage, which reduces their effectiveness. Capacitive voltage sensing methods also add system cost due to the need for high voltage capable capacitors.